The hoofs of ungulate animals comprise a hard shell surrounding and secured to a sole by a white layer or white line and a frog substantially enclosed within the sole. The shell is equivalent to human nails in that it continually grows lengthwise so that its lower edge extends below the bottom surface of the hoof and protects the relatively soft and sensitive sole and frog. In normal use the bottom edge of the horn or shell is worn away by traveling over the terrain and in this process as the weight of the animal is imposed upon and then removed from the hoof, the horn expands and contracts, or at least the sidewall portions thereof, which transmits such movement through the sole to the frog to impart to the frog the necessary movement to function as a pump for forcing blood through the animals legs and the hoof.
Upon domestication of the horse and more particularly the development of hard surface roads and streets, the horn or shell was worn away by the constant friction with such hard surfaces to the extent that it did not provide sufficient protection to the sole and frog. To overcome this problem, shoes were devised of iron and to attach such shoes to the hoof nails were passed through the shoe and driven through the horn to hold the shoe in place. While the iron maintains the sole and frog above contact with the surfaces over which the animal travels, it inhibits the fluctuation of the horn to provide the pumping action for the frog so that the animals frequently develop hoof problems. Moreover, unless the blacksmith is skilled in applying the shoes, the nails engage the sensitive sole and produce soreness of a nature entirely foreign to the natural operations of the hoof. Finally, due to the effort of the horn to fluctuate, the nails engaged in the horn in due course wear loose and the shoe can be lost. Also, it is necessary periodically to replace the shoes in order to remove the excess portion of the horn because of its continuous growth.